As illustrated in FIG. 1A, local contact opening (LCO) lines 102 are formed on the back surface of the solar cell substrate of a common solar cell. A common solar cell is a PERC (passivated emitter and rear contact) cell design, laser fired contact (LFC) cell design or bifacial cell design with rear side contact grids. The LCO lines are laser cut trenches that are formed directly in the solar cell substrate, which is commonly consisting of silicon, or an interlayer, which is usually consisting of alumina, silicon nitride, silicon oxynitrid or another passivating material. Trenches are filled with a metal, which is usually aluminium. The metallization of the trenches may be formed by means of a screen printed metal containing paste. The solar cell with the paste is fired to form a metal in the trenches. The fired aluminum may flow to the end of the trenches such that aluminum pearls are formed. These pearls may be formed on every laser line. So far, there is no known solution to prevent the formation of these pearls. The bumps and point loads of these pearls may lead to an increase of the fracture rate of the solar cell. This may cause a breakage and cracking of the solar cells and may thus reduce the lifetime of solar modules.
Alternatively, the metallization of the LCO trenches may be achieved using any of the following examples: plating, vapor deposition techniques (e.g. PVD, CVD), and atomic layer deposition (ALD).
In a conventional solar cell, the LCO lines 102 are formed parallel to at least one edge of the solar cell substrate. Moreover, a conventional solar cell silicon substrate is formed such that the {110} crystal orientation is parallel to planar surface of the solar cell substrate. Therefore, the energetically preferable {111} crystal direction 112 of the solar cell silicon substrates is non parallel to the LCO lines 102.
The periodic orientation of LCO lines, and further of Ag pad structures, grid structures, Laser contacts of the solar cell, parallel to a crystal orientation of the solar cell substrate, e.g. parallel to the {110} crystal orientation in a quasi-monocrystalline PERC cell, may support the propagation of cracks in the solar cell substrate, e.g. in the {111} direction 112 of the solar cell substrate. Typically, the contact structures extend continuously or discontinuously parallel to crack propagation directions in the crystalline solar cell substrate. Local mechanical tensions of these periodically repeating continuous structures may add up to a larger linear tension line in the direction of a crystal orientation of the solar cell substrate. These linear tension lines may act like a perforation of the solar cell substrate and may reduce the stability of the solar cell parallel to LCO lines or solder pads. Therefore, a crack may propagate almost unhindered along the tension line without leaving this energetically preferred direction, e.g. the energetically preferred {111} direction 112 of the solar cell substrate. In other words, the periodic repetition of contact structures, e.g. bus bars, solder pads, grid finger, LCO on a solar cell may lead to a superposition of tension fields of single contact structures and may thus form tension lines. The extension of these tension lines may have a preferred direction in the solar cell substrate since this is anisotropic. In some solar cell designs, the direction of the tensions lines may be parallel to the {111} main crack propagation direction in a crystal direction of a silicon substrate,
Further, in the processing and testing of a conventional solar cell module, an increase of the fracture rate and probability of crack formation is observable due to the decrease of solar cell substrate thickness, solar cell substrate crystal orientation and novel solar cell design concepts, e.g. in {110} quasi-monocrystalline PERC cells. Solar cells are usually exposed to various stress factors while processed, e.g. temperature stress during soldering; applying solar cell connectors to a solar cell with a different temperature expansion coefficient than the solar cell substrate; transport and handling of soldered solar cells, strings, laminates and modules, lamination of a solar cell or solar cell module by means of pressure and temperature stress. Further stress factors may be applied to a solar cell or solar cell module during the operating time, e.g. altering environmental temperature, weather inflicted mechanical stress, e.g. by wind and snow. These stress factors may partially be reduced in the processing by compensational means, e.g. altering the processing conditions, e.g. the lamination process, adaptation of soldering tips positions, use of double layered ethylene vinyl acetate (EVA) encapsulation. However, a complete reduction of stress factors might not be possible in the processing and operating time of the solar cell and solar cell module due to the solar cell design. The reduction of the fracture rate may be insufficient and mentioned solutions to reduce the fracture rate may be cost intensive and time consuming.
Moreover, the above described effects may become critical if multiple LCO lines are compromised. This may increase the probability of cracking of the solar cell, e.g. in {110} quasi-monocrystalline solar cells and {110} quasi-monocrystalline PERC cells. In this case, the main crack propagation direction may be parallel to the direction of the periodically repeating contact structures or to the direction of the extension of linear shaped contact structures.
In a conventional solar cell structure, the LCO lines 102 are paired with a full rear side metallization layer 114, which is typically aluminum. Solder pads 116 may be electrically connected with the solar cell substrate to provide an electrical connection access to the solar cell by printing these pads with firing solder paste. Alternatively, solder pads 116 may be printed with non firing paste. Those pads will remain on dielectric layer. The metal filled trenches 102 provide a contact between the solar cell substrate (cell base) and the solder pads 116 of the solar cell. The solder pads 116 are formed of, on or in the rear side metallization layer 114 on the LCOs 102. Further, a conventional solar cell may include solar cell bus bars on the solar cell substrate to increase the current distribution in the solar cell substrate. Conventional LCO lines 102 run perpendicular to solar cell bus bars and solder pads 116 (as illustrated in FIG. 1B).
Alternatively, the solder pad could be performed e.g. using any of the following deposition processes: plating, vapor deposition (e.g. CVD, PVD), and atomic layer deposition (ALD).
One disadvantage of this design usually is a power degradation, e.g. in the PERC technology, due to solder pad 116 deactivation if an LCO line 102 is interrupted or its connection to the solder pad 116 is lost. The cell current has to flow through the silicon base material to the next LCO line 102 resulting in a higher series resistance and a cell power loss. Further, the series resistance may increase during temperature cycling in the processing of the solar cell. Moreover, the full area rear metallization layer 114 may lead to a bow of the solar cell after the firing process and cooling down of the solar cell. The bow may be due to internal stresses between the layers e.g. between solder pads 116 made of silver in a metallization layer 114 made of aluminum. The stress may be caused by significantly different thermal expansion coefficients with respect to the solar cell substrate (CTE(Al): 23.1 ppm/K, CTE(Ag): 18.9 ppm/K, CTE(Si): 2.6 pp/K). The stresses created by a full back side metallization layer 114 can increase the likelihood of an LCO line 102 being compromised as described above. Further, the stresses may promote the formation of cracks within the solar cell substrate, e.g. in solar cell substrates formed of a brittle material, e.g. silicon.